In traditional papermaking processes, water is introduced to the paper making system as a carrier fluid. The carrier fluid is used to suspend wood fibers, suspension aids, filler materials, and the like as a slurry, which is spread evenly over a wire web. The carrier fluid is drained, squeezed (e.g., using nips), and vacuumed away from the web, while most of the fillers, fibers, and retention aids remain on the web and eventually form the paper sheet. The water that is drained away from the paper sheet is called whitewater, due to the high residual fiber, filler, and brightener content, which imparts a white color to the water in some cases.
Papermaking processes generally utilize large amounts of water, with typical usages ranging from 200 gallons per ton of paper for highly recycled paper board to 30,000 gallons per ton of paper for specialty fine paper. In the early days of paper making, fresh water was routinely consumed from rivers and lakes to produce the paper. However, such large amounts of fresh water consumption were harmful to the environment. To limit environmental damage, there has been a movement to reduce fresh water use and reduce wastewater discharges from pulp and paper manufacturing facilities.
Current pulp and paper mills implement numerous whitewater reuse strategies. Most of these strategies clean the recycled wastewater using simple suspended solids clarification, filtration, and screening techniques. The reduction in fresh water make-up to the paper machine has resulted in a substantial increase in concentration of the contaminants in the whitewater system. The recovered water is generally used many times before it is finally discharged. Simple suspended solids clarification, filtration, and screening techniques do not address the increase in dissolved and colloidal solids in the whitewater loops. In many current systems, chemical additives are fed to the whitewater stream in an attempt to control organic material, colloidal solid, and dissolved ion accumulation. However, many typical reuse strategies are unable to effectively remove fine colloidal substances, total dissolved solids (including scale causing ions), and foulants such as iron aluminum and bacteria. As a result, a significant amount of dissolved and colloidal substances introduced by the paper process are allowed to cycle up in concentration as the whitewater is continuously reused and as fresh water make-up is reduced. This can cause numerous problems in the paper machine, paper product, and in other areas of the mill. For example, as sparingly soluble salts cycle up in concentration, they may eventually precipitate and scale. As colloidal anionic particles (commonly referred to as anionic trash) cycle up in concentration, they may inhibit the flocculation and removal of suspended solids, leading to the fabrication of an inferior paper product. In addition, the buildup of total dissolved solids (TDS) and especially anionic colloidal trash adversely affects the performance of retention aids, which are chemical additives that promote the retention of fibers on the paper web and dewatering of the paper product when the wet slurry is undergoing the dewatering process. If the fibers on the web do not dewater properly, drainage aids are sometimes applied. The increased contaminant load in the whitewater reduces the effectiveness of the drainage aids. As a result, the paper sheet in production has a higher moisture content, which can increase the amount of heat energy that is required to be used to dry the paper in the dryer section of the paper machine. The energy increase can be substantial and costly.
Two basic techniques are primarily utilized to control the buildup of sparingly soluble salts and anionic colloidal substances. One such technique involves chemical addition. For example, sequestration chemicals can be added to complex or chelate multivalent cations so they can no longer easily bind with counter ions such as sulfate and carbonate to form scale. Another technique involves chemical softening. This technique is based on the principal that raising the pH of a solution in the presence of alkaline ions brings the sparingly soluble salts to their saturation point. This causes the salts that are present to precipitate as a solid for subsequent removal by sludge dewatering systems.
The addition of chemical additives to process whitewater can cause several problems. For example, continuous addition of chemical additives is necessary to control scaling and fouling. Continual addition of chemical additives is also needed to compensate for increasing concentrations of contaminants in the whitewater loops. The chemicals are in part charge neutralized by the increased contaminant load in the whitewater. The excess chemical consumption is costly to the mill and causes a buildup of total dissolved solids (TDS), which make the whitewater difficult to treat when it is finally discharged. The buildup of chemical additives and background salts has a significant negative effect on mill production, sheet quality, and consumption of heat energy, as described above. Chemical additives that are used in traditional whitewater recycling processes are also consumed in part by the anionic trash within the whitewater, as the anionically charged colloids attach to the cationic chemical additives. Because the anionic trash consumes the chemical additives, the chemical additives must be added in excess, which can increase the degree to which additives accumulate in the process. Systems and methods for whitewater clarification that do not involve the addition of large amounts of chemical additives would be desirable.